Simplified registration image display screen

ABSTRACT

The invention relates to image display screens using a cell matrix to form an image. More particularly, the invention relates to means for facilitating relative positioning of various elements during manufacturing. The inventive screen consists of at least two electrode networks. One of the characteristics of the invention is that at least one of said networks consists of “variable direction” electrodes which are shaped in such a way that they spread out and then return towards their longitudinal axis to intersect and pass alternatively from one side to the other of said axis. The spread of a variable direction electrode in relation to the longitudinal axis has an amplitude depending on the position of the electrode in respect to a reference position. This arrangement provides appropriate dimensional leeway to facilitate superpositioning of several masks of varying dimensions. The invention is used in “flat screens”, specially plasma panels.

FIELD OF THE INVENTION

The present invention relates to image display screens of the << flatscreen >> type. It relates more particularly to means used to facilitateand simplify operations for the positioning of the constituent elementsof these screens.

BACKGROUND OF THE INVENTION

There are different types of image display screens that come under thecategory of flat screens, for example plasma panels, liquid crystaldisplays, screens whose cells use a “point effect” phenomenon to producean electron beam each or again light-emitting diode screens.

These different flat screens have the common feature of having a matrixstructure: to each elementary dot of the displayed image therecorresponds a cell (or even several cells in the case of color images)and each cell is defined substantially at the intersection of two ormore arrays of electrodes. Consequently, the manufacture of thesedifferent types of flat screens entails the same difficult problem foreach of them, a problem that lies in the difficulty of registering thedifferent elements used to form a cell, namely the difficulty ofpositioning all these elements with respect to one another and in thesame way for all the cells of the screen.

The following explanations, given with the example of plasma panels(abbreviated as PP hereinafter in the description), will provide for aclearer understanding of the importance of the above-mentioned problemof registration.

PPs work on the principle of an electrical discharge in gases. Theygenerally comprise two insulating plates each bearing one or more arraysof electrodes and mutually demarcating a gas-filled space. The platesare assembled with respect to one another so that the arrays ofelectrodes are orthogonal. Each intersection of electrodes defines acell to which there corresponds a gas space.

FIG. 1 shows the structure of an alternating color PP of the type usingonly two intersecting electrodes to define and control a cell asdescribed especially in the French patent application published underNo. 2 417 848.

The PP has two substrates or plates 2, 3. One of them is a front plate2, namely the one that is on an observer's side (not shown). It has afirst array of electrodes called “row electrodes” of which only threeelectrodes Y1, Y2, Y3 are shown. The second plate 3 forms the rearplate. It is opposite the observer and therefore it is this plate that,preferably, is provided with elements that can prevent the transmissionof light to the observer. It has a second array of electrodes called“column electrodes”, of which only five electrodes X1 to X5 are shown.The two plates 2, 3 are made of the same material, generally glass.These two plates 2, 3 are designed to be joined to each other so thatthe arrays of row and column electrodes are orthogonal with respect toeach other.

It is common practice that, in the front plate 2, as in the exampleshown, the row electrodes Y1 to Y3 should be separated from one anotherby black strips 4 (forming what is called a “black array”) designed toimprove the contrast between cells of different rows. The row electrodesY1 to Y3 are covered with a layer 5 of dielectric material by which theyare insulated from the gas.

On the rear plate 3, the column electrodes X1 to X5 are also coveredwith a layer 6 of dielectric material. The dielectric layer 6 is itselfcovered with layers forming strips 7, 8, 9 of luminophor materialsrespectively corresponding in the example to the colors green, red andblue. The luminophor strips 7, 8, 9 are positioned in parallel to thecolumn electrodes X1 to X5, above these column electrodes from whichthey are separated by the dielectric layer 6. The rear plate 3furthermore has separation barriers 11 that are parallel to theluminophor strips 7, 8, 9 and separate these strips from one another.

The PP is formed by the joining of the front and rear plates 2, 3. Thisjoining sets up a matrix of cells. The cells are then defined each atthe intersection between a row electrode Y1 to Y3 and a column electrodeX1 to X5 with a pitch P1 parallel to the row electrodes that is given bythe distance between the column electrodes and with a pitch P2 along thecolumn electrodes that is given by the distance between the rowelectrodes. Each cell has a discharge zone whose section correspondssubstantially to the facing surface of the two crossed electrodes. Ineach cell, the discharge into the gas generates electrical charges andin the case of a “alternating” PP, these charges collect at thedielectrics 5, 6 facing the row and column electrodes. In the exampleshown, this operation is obtained by means of recesses Ep1 to Epn madein the luminophor strips 7, 8, 9 substantially on the useful surfaces ofthe column electrodes X1 to X5, namely the surfaces of these electrodesthat define the section of the discharge zone.

Thus, in the example shown, the intersections made by the first rowelectrode Y1 with the column electrodes X1 to X5 define a row of cells,each cell being represented by a recess: the first cell C1 is located atthe first recess Ep1, the second cell C2 is located at the second recessEp2 and so on and so forth until the fifth recess Ep5 which represents afifth cell C5. The first, second and third recesses Ep1, Ep2, Ep3 arerespectively located in a green luminophor strip 7, a red strip 8 and ablue strip 9. They thus correspond to monochromatic cells having threedifferent colors which, in a set of three, may constitute a coloredcell. Thus, for 1024 colored cells per row for example, the plate 3 mustcontain 1024 times per line the above-described structure. The columnelectrodes X1 to X5 have a width Lg1 of about 50 microns and theirlongitudinal axes are spaced out for example by 250 microns. This givesan idea of the difficulties of manufacture, especially for obtaining anaccurate position of the recesses Ep1 to Epn.

The operating quality of the PP depends on the geometrical anddimensional characteristics of the cells, and hence on the quality ofregistration which is defined as the precision of the positioning, withrespect to one another, of its elements such as the row electrodes andthe column electrodes, the barriers 11 and the recesses Ep1 to Epn forwhich, in particular, the required precision of registration may be inthe range of ±20 ppm (20 parts per million), i.e. for example 10 μm.

Such precision is very difficult and hence very costly to obtain in thecontext of industrial-scale manufacture. Indeed, the manufacture on aplate 2, 3 of the different elements referred to here above makes use inparticular of the technique of photographic masks used on photosensitivelayers and/or techniques of silk-screen printing. For the rear plate 3for example, after the array of column electrodes X1 to X5 has beenformed and then the dielectric layer 6 has been deposited, thelumionophor strips 7, 8, 9 are deposited on this dielectric layer 6.Then, the recesses Ep1 to Epn are made in the luminophor strips, alongwith the separation barriers 11, with all the precision possible. Themasks used to define the different patterns such as electrodes,recesses, etc. furthermore comprise, in a standard way, specificalignment or positioning patterns used to align elements to be made withthose obtained at a previous level or stage of manufacture. It must benoted that the term “mask” is used to designate both photographic masksand silk screens.

FIGS. 2a, 2 b show alignment patterns Ma1, Ma2 of this kindcorresponding in the example respectively to a mask 20 for thedefinition of the recesses Ep1 to Epn or a mask 21 for the definition ofthe column electrodes X1 to X5. These alignment patterns consist ofregistration patterns along the two axes X and Y and conventionally theyare located outside a useful surface S1, S2 bearing the drawing (notshown) of the elements to be defined.

The alignment pattern Ma1 (FIG. 2a) has the general shape of a T formedby a horizontal aperture Oh and a vertical aperture Ov. FIG. 2b showsthe alignment pattern Ma2: it comprises firstly three vertical referencemarks R1, R2, R3 corresponding for example respectively to the columnelectrodes X1, X2, X3 and secondly a horizontal reference mark Rh. Todefine the position of the recesses with respect to one of the columnelectrodes, the electrode X2 for example, it is enough to place the mask20 bearing the recesses so that the apertures Oh and Ov of the alignmentpattern Ma1 are centered respectively on the horizontal reference markRh and the vertical reference mark R2.

Naturally, in order that the quality of the positioning of the recesseswith respect to the electrodes should be the same for the entire usefulsurface S1, SZ these two masks 20, 21 should be perfectly matched.

The precision needed for the positioning of the recesses Ep1 to Epn withrespect to the column electrode X1 to X5 is of the greatest importance.It may be required to within plus or minus some tens of ppm and ofcourse this precision is required for the mask used to define elements.It is therefore not possible, when such precision is sought to useconventional masks for example of the type made with gelatin on a Mylarsupport whose cost is not very high, for masks of this type havedimensional variations of more than 10 ppm per 0° C. as well as perpercentile point of hygrometry. In addition to this, there is alsoimprecision due to tracing conditions.

The manufacturers therefore, in making these masks, have been led to useglass-based substrates with very high dimensional stability. However,these substrates have the drawbacks, in particular, of being limited insize and having a very high cost. Their use entails particularly heavypenalties when they are used to obtain insolation by contact for then,despite their high cost, they soon get damaged.

Another difficulty in making such registration arises out of thedimensional variations of the plate 2, 3 when it is subjected to heattreatment. The glass plates 2, 3 undergo a heat treatment effect thatcomes into play between the making of the electrode arrays and that ofthe luminophor strips 7, 8, 9 or separation barriers 11. The temperaturereached is in the range of 580° C, i.e. the softening point of glass.Upon return to the ambient temperature, the plates 2, 3 show majordimensional variations (in terms of shrinkage and compaction). Thesevariations are difficult to take into account with a view toregistration for they are not reproducible to a precision of within morethan a few tens of ppm, especially with ordinary sodium-calcium typeglass.

These explanations show the seriousness of the problem raised by theregistration of the different constituent elements of an alternating PPincluding the registration made necessary by the assembling of the frontand rear plates, in particular when these two plates each bearelectrodes as is most usually the case. It must be noted that theseproblems exist in a manner that is quite similar for the other types ofPP and more generally for all the flat image display screens, providedthat, like the PP, they comprise a matrix of cells each controlled bymeans of at least two crossed electrodes.

The present invention is aimed at facilitating the registration of thedifferent elements of the matrix structure display screens. It makes itpossible to avoid the various drawbacks mentioned here above, andespecially to overcome constraints imposed by differences in dimensionbetween masks and/or between a mask and an already obtained level ofelements.

To this end, the invention proposes to provide at least certainelectrodes of at least one array with a shape such that it gives adimensional latitude for example of about hundred ppm or even more andthus makes it possible to compensate for the dimensional differenceswhich are detrimental to the quality of registration.

SUMMARY OF THE INVENTION

According to the invention, there is proposed an image display screencomprising a matrix of cells, at least two electrode arrays, theelectrodes of one array being orthogonal to the electrodes of the otherarray and each cell corresponding to an intersection of electrodes,wherein at least one electrode array comprises so-called variabledirection electrodes each positioned along a longitudinal axis andhaving a shape such that each one diverges from and then approaches itslongitudinal axis to intersect it and pass alternately on either side ofthis axis and plot a repetitive pattern, the divergence shown by avariable direction electrode with respect to the longitudinal axishaving an amplitude as a function of the position of the electrode withrespect to a reference position, this divergence varying from oneelectrode to another.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be understood more clearly from the followingdescription, made by way of a non-restrictive example with reference tothe appended figures, of which:

FIG. 1 already described shows a standard structure of a matrix typeimage display screen;

FIGS. 2a, 2 b which are already described show standard registrationpatterns used for the positioning of the masks,

FIG. 3 gives a schematic view of an array of electrodes according to theinvention;

FIGS. 4a, 4 b show patterns of registration of masks compatible with anarray of electrodes according to the invention.

DETAILED DESCRIPTION

FIG. 3 gives a simplified view of an array of electrodes RE of an imagedisplay screen of the matrix structure type according to the invention,for example an alternating PP similar to the one shown in FIG. 1. Thearray of electrodes according to the invention may be for example anarray of column electrodes fulfilling the same function as theelectrodes X1 to X5 of FIG. 1 which may also be borne by a support 3 asuch as the rear plate 3 of FIG. 1.

According to one characteristic of the invention, this array of columnelectrodes comprises so-called variable direction electrodes E1 to En,E′1 to E′n, called variable electrodes hereinafter in the description(in the example shown, the number n of variable electrodes is equal to 6but of course in practice this number may be is greater, severalthousands for example); the other electrodes of this array RE have aconventional shape and are positioned along longitudinal axes Ax and arereferenced X1 to X5.

Each variable electrode extends along an axis called a mean longitudinalaxis Al. In the non-restrictive example described, its shape is suchthat it is formed by a sequence of zigzag lines that intersect the meanlongitudinal axis Al and pass alternately on either side of this axis.The longitudinal axes Ax are separated from one another by a distance d1and separated by a distance d2 from the mean longitudinal axes Al, thelatter being themselves separated from one another by distances d3 tod7. In a first embodiment of the invention, these distances d1 to d7 aresubstantially the same.

The to-and-fro motions or excursions of each variable electrode E1 toEn, E′1 to E′n with respect to its mean longitudinal axis Al createpatterns M1 whose repetition corresponds to a pitch P3 which must besubstantially the same as (or a submultiple of) the one according towhich the cells (not shown) will be formed along the mean longitudinalaxes Al. In other words, if we take for example the PP of FIG. 1, thepitch P3 of these patterns must substantially correspond to the distancebetween the axes of the row electrodes Y1 to Y3.

The maximum excursion accomplished by each variable electrode E1 to En,E′1 to E′n on either side of its mean longitudinal axis Al isrepresented in FIG. 3 by the divergence or difference D1 to Dn, D′1 toD′n. This divergence is presented at each pattern MI between the meanlongitudinal axis Al and an axis of symmetry As dividing the width ofthe tracks of each of these electrodes into two. Naturally, the generalshape defined here above of the variable electrodes E1 to En, E′1 to E′nmay be obtained in different ways, for example by making the electrodesfollow a path with a sinusoidal shape.

According to another characteristic of the invention, for each variableelectrode E1 to En, E′1 to E′n, the values of divergence D1 to Dn andD′1 to D′n have an amplitude that may vary as a function of the positionof the electrode with respect to a reference position.

In the non-restrictive example shown in FIG. 3, the reference positioncorresponds to a column electrode X3 called a central electrode, that issubstantially rectilinear, as well as the four column electrodes X1, X2,X4, X5 in the middle of which it is placed. The central electrode X3takes up a central position in a useful zone Zu representing the surfacearea occupied by all the electrodes on a support such as the plate 3 a.The variable electrodes E1 to En located between the straight electrodesX1 to X5 and an end of the useful zone Zu close to an edge 15 of theplate 3 a show values of divergence D1 to Dn that can range from anamplitude Amin which is the smallest amplitude for D1 to an amplitudeAmax which is the greatest amplitude for the divergence Dn thatcorresponds to the electrode En at the greatest distance from thesestraight electrodes.

Symmetrically, there is a similar organization to the left of thestraight electrodes X1 to X5 with variable electrodes E′1 to E′n showingvalues of divergence D′1 to D′n that can range (for example with thesame values as in the previous case) from the lowest amplitude Amin tothe highest amplitude Amax for the electrode E′n which is the closest toan edge 16 opposite the first edge 15.

The advantage of a configuration of this kind is that, perpendicularlyto the variable electrode axes Al, by translation along these axes onthe length L5 of a pattern M1, it offers a variable value at a distanceDL defined between the two end electrodes En and E′n, this distance DLbeing capable of forming a row of elements such as recesses with a viewto forming cells. The distance DL is made variable within limits givenby the maximum amplitude Amax of the values of divergence Dn, D′n.Indeed it can be observed that:

on a straight line perpendicular to the mean longitudinal axes Al andgoing through points where the variable electrodes intersect these axesAl, the far edges of the two variable electrodes En, E′n positioned atthe opposite ends of a useful zone Zu are separated by a length L1corresponding to a standard dimension, namely the same as in the casewhere all the electrodes are straight;

on another straight line parallel to the length L1 and intersecting themean longitudinal axes Al at the points where the variable electrodesare at the greatest distance from their longitudinal axes, towards theexterior of the useful zone Zu, the far edges of the two variableelectrodes En, E′n positioned at the opposite ends of the useful zone Zuare separated by a second length L2 greater than the first length L1,namely equal to L1+2Dn;

on another straight line parallel to the length L1 and intersecting themean longitudinal axes Al at the point where the variable electrodes areat the greatest distance from their longitudinal axis, towards theinterior of the useful zone Zu, the far edges of the two variableelectrodes En, E′n positioned at the opposite ends of the useful zone Zuare separated by a third length L3 that is smaller than the first lengthL1, namely equal to L1−2Dn.

A configuration of this kind therefore makes it possible to compensatefor a difference in dimension between the plate 3 a bearing theelectrodes such as the one described here above and a mask used todefine the additional elements which are made at a subsequent stage.This configuration makes it possible in particular to optimize thesuperimposition with a mask used to define recesses Ep1 to Epn (shown inFIG. 1) by simple translation along the electrodes. The maximumdivergence that can be compensated for, counted for example between thecentral electrode X3 and one of the end electrodes En, E′n, correspondsto the maximum amplitude Amax of a divergence, this maximum amplitudepossibly reaching a hundred or many hundreds of ppm.

FIGS. 4a, 4 b show patterns of alignment Ma1′, Ma2′ of masks 20′, 21′respectively adapted, on the basis of the alignment patterns Ma1, Ma2 ofFIGS. 2a, 2 b, for use with an array RE of electrodes according to theinvention.

FIG. 4b shows the alignment pattern Ma2′: it has three verticalreference marks R1, R2, R3 and the horizontal reference Rh alreadydescribed with reference to FIG. 2b plus an additional alignment elementmc2. This element has three drawings 22, 23, 24 side by side eachpartially reproducing a track of a variable electrode E1 to En. Thesedrawings are positioned in parallel to the reference marks R1, R2, R3.

The alignment pattern Ma1′ (FIG. 4a) has the horizontal aperture Oh andthe vertical aperture Ov (already described with reference to FIG. 2a)plus an additional pattern mc1 formed by two apertures O1, O2. These twoapertures are positioned on one and the same axis Ao parallel to thevertical aperture Ov and their centers are substantially distant by oneand the same length L5 as that of a pattern M1.

Thus, with the alignment pattern Ma2′ being transferred to the rearplate 3 a during the making of the electrodes, it is enough, for theaccurate positioning of the masks 20′ bearing the recesses, to obtain acoincidence between the alignment patterns Ma1′, Ma2′ and then translatethe mask 20′ in parallel to the reference marks R1, R2, R3 up to thetime when the two apertures O1, O2 are fully above a track of a drawing22, 23, 24.

Reference is made again to FIG. 3, in the case for example of an arrayof column electrodes of PP such as the one shown, the plate 3 a bearingfor example 1024 electrodes, with straight electrodes in the centralpart such as the electrodes X1 to X5 and on each side variableelectrodes E1 to En. The electrode tracks all have one and the samewidth equal for example to 100 μm and the distances d1, d2, d3 betweenaxes Ax, Ad of electrodes are the same, for example 0.5 millimeters.Thus, a length L3 of the useful zone Zu is about 520 millimeters and thevalue of a hundred ppm referred to here above corresponds to about 52micrometers.

For a divergence Dn having a maximum amplitude conferred on a variableelectrode En, E′n at the greatest distance from the electrode which isthe positional reference, each intermediate variable electrode E′1 toE′5 may show a divergence D1 to D5 that gradually increases as and whenthe electrode moves away from the reference position. Assuming forexample that the variable electrodes E1 to En located towards the firstedge 15 are separated from the variable electrodes E′1 to E′n locatedtowards the second edge 16 by a single straight electrode X3 used as apositional reference, the variation of the amplitude of the divergencebetween one variable electrode and the next one may be equal to thevalue of the smallest amplitude Amin. The smallest amplitude Amincorresponds to the amplitude of the biggest divergence Amax, divided bythe number N.Ev of variable electrodes, giving Amin=Amax/N.Ev. Thus, inthis example, with values of divergence Dn, D′n having the greatestamplitude Amax: the values of divergence D1, D′1 would have the smallestamplitude Amin; the values of divergence D2, D′2 would have theamplitude Amin×2; D3, D′3 would have the amplitude Amin×3, D4, D′4 wouldhave the amplitude Amin×4; D5, D′5 would have the amplitude Amin×5.

However, given limits dictated by the means for the manufacture of masksfor electrodes, by tracing means in particular, such gradualness of theamplitude variations of the values of divergence may be difficult toobtain. It is then possible to obtain a variation in the value of thevalues of divergence D1 to Dn not with each variable electrode E1 to En,E′1 to E′n but by groups of these electrodes. Indeed, rather thanmodifying, at each variable electrode, the amplitude of the divergenceby a low value that is difficult to ensure, it is possible to assign, toN consecutive variable electrodes, one and the same amplitude ofdivergence and then for the N consecutive electrodes that follow, toincrease their amplitude of divergence by a value N that is N timesgreater.

This possibility is illustrated in FIG. 3 where the different electrodesX1, X2, X3, X4, X5, E1 to En, E′1 to E′n that constitute the array ofcolumn electrodes form groups G1, G2, G3, G4, G′1, G′2, G′3, G′4.

The first group Gl positioned to the right of the central electrode X3is represented by two straight electrodes X4, X5. Then, after G1, thereis a second group G2 formed by two variable electrodes E1, E2 showingvalues of divergence D1, D2 of the same amplitude and then a third groupG3 formed by variable electrodes E3, E4 showing values of divergence D3,D4 of the same amplitude and finally a fourth group G4 comprising thevariable electrodes E5, En whose values of divergence D5, Dn also havethe same amplitude. To the left of the central electrode X3, there is asame organization: namely a group G′1 of two straight electrodes X2, X1followed by a group G′2 of two variable electrodes E′1, E′2 showingvalues of divergence D′1, D′2 of the same amplitude, then a group G′3formed by variable electrodes E′3, E′4 showing values of divergence D′3,D′4 of the same amplitude and finally a last group G′4 comprising thevariable electrodes E′5, E′n whose values of divergence D′5, D′n alsohave one and the same amplitude.

In this configuration where all the variable electrodes belonging to oneand the same group have a divergence of the same amplitude, this commonamplitude Ac may be determined for each of the groups of electrodes bymultiplying the minimum amplitude Amin by the number N.E.P. ofelectrodes positioned between the group considered and the centralelectrode X3 and then adding Amin, that is to say by applying thefollowing relationship:

Ac=(Amin×N.E.P)+Amin

By applying it to the example of the groups G1, G2, G3, G4 shown in FIG.3, to the right of the central electrode X3 (however, it is equallyvalid for the groups located to the left of this central electrode) andif the electrodes E5, En of the group G4 have values of divergence D5,Dn of one and the same amplitude which is the highest amplitude Amax:the two electrodes E1, E2 of the second group G2 have values ofdivergence D1 and D2 of one and the same amplitude equal to(Amin×2)+Amin, giving 3 Amin; the two electrodes E3, E4 of the thirdgroup G3 have values of divergence D3, D4 whose amplitude is equal to(Amin×4)=Amin, giving 5 Amin.

Naturally, in practice, each group may contain a greater number ofelectrodes than in the example shown so that the increase in theamplitude of the values of divergence from one group to the next groupis sufficiently significant to be obtained by the tracing means. Forexample, if the variation in amplitude of the divergence from onevariable electrode to the next variable electrode should be 0.635micrometers (giving Amin=0.635 μm), it is easier to give a sameamplitude of divergence to ten consecutive electrodes and then increasethis amplitude by 6.35 micrometers for the next ten variable electrodes.Thus, in the example of FIG. 3, each group may be formed by N electrodeswith one and the same amplitude of divergence in each group, anamplitude which for example would be successively 6.35 μm, 12.7 μm,19.05 μm, etc. for the successive groups G2, G3, G4, namely with jumpsof 6.35 μm from one group to the other.

The value of the greatest amplitude of divergence Amax is determined soas to enable a compensation for dimensions, especially in order toobtain an accurate superimposition of a mask on electrodes formed on aplate after a plate-electrode assembly has undergone heat treatment(annealing). In such a case, the way in which the dimensions produced bytreatment vary is generally known but the value of the variation isdifficult to foresee. It is therefore the lack of reproducibility (ofplus or minus 50 ppm or even more in the case of a sodium-calcium typeglass) that raises particularly great problems.

Thus, when the way in which the variation takes place can be foreseen,it is also possible to adjust the length L4 of the useful zone Zu as afunction of the average rate of shrinkage caused by the thermaltreatment (annealing). To this end, the invention proposes, incombination with the shape of the variable electrodes E1 to En, E′1 toE′n, to make use of the distances d1 to d7 between axes of electrodes oron some of these distances by increasing them or reducing them in orderto increase or reduce the useful zone Zu depending on the way in whichthe variation is expected to take place. This embodiment thereforeconsists, for example, in order to increase the length L4 of the usefulzone Zu:

either in increasing the distance between the electrodes, namely thedistance between the longitudinal axis of one electrode and thelongitudinal axis of a following electrode, starting from the centralelectrode X3 and going up to an end electrode En, E′n by gradualincreases: the distance d7 between the electrodes E5 and En is thengreater than the distance d6 between the electrodes E4 and E5;

or by acting on these distances by groups G1, G′1, G2, G′2, G3, G′3, G4,G′4 of electrodes. In this case, in taking for example the straightelectrodes of the central electrode X3: all the mean longitudinal axesAl of the variable electrodes E1, E2 of the group G2 may undergo arightward shift by 6.35 μm (these axes are then referenced Al2); themean longitudinal axes Al of the variable electrodes E3, E4 of the groupG3 undergo a rightward shift by 12.7 μm (these axes are then referencedAl3). The mean longitudinal axes Al of the variable electrodes E5, En ofthe group G4 undergo a rightward shift by 19.05 μm (these axes are thenreferenced Al4).

It must be noted that, for variations or differences in dimensions thatmight require compensation in a direction opposite the one token hereabove as an example, it is sufficient to act in reverse: what must bedone then for example is to give the maximum amplitude Amax to thevalues of divergence D1, D′1 closest to the reference position, namelythe central electrode X3 and to give the lowest amplitude Amin to themost distant values of divergence Dn, D′n. Similarly, the modificationof the length L4 of a useful zone Zu can be accomplished as a reductionby acting on the distances between electrodes so as to give a greatervalue to the distance d3 between the electrodes E1 and E2 than to thedistance d7 between the electrodes E5 and En.

It must be further noted that the reference position constituted in theabove example at a central position by the central electrode X3 may belocated at a different position, for example at one of the ends of theuseful zone Zu.

The reference position at the central position enables the distribution,on either side of this position, of a difference in dimensions forexample between the embodiment of the electrodes on the plate 3 a and arecess mask to be superimposed on these electrodes. In other words, themaximum amplitude Amax of a divergence Dn may in this case have a valuewhich is half that of the difference in dimensions. On the contrary,should the reference position be located at one end of the useful zone,the maximum amplitude Amax must correspond to the entire value of thedifference in dimensions.

The invention can also be applied advantageously to the manufacture ofan array of row electrodes such as the electrodes Y1 to Y3 borne by thefront plate 2 shown in FIG. 1. In this case, the invention would make itpossible, here too, to obtain a latitude in dimensions which inparticular would facilitate the positioning of the front plate withrespect to the rear plate during the assembly of the two plates.Naturally, if a black contrast-improving array (shown in FIG. 1) ispositioned between the row electrodes, the strips 4 of this black arraywould follow the contour of the electrodes in order to be self-centeredlike these electrodes.

Thus, as indicated here above, the invention can be applied in a mannersimilar to that described here above, not only in the other types ofplasma panels but also in the other types of image display screensimplementing a matrix of cells to form the image.

What is claimed is:
 1. An image display screen comprising a matrix ofcells, at least two electrode arrays, the electrodes of one array beingorthogonal to the electrodes of the other array and each cellcorresponding to an intersection of electrodes, wherein at least oneelectrode array comprises so-called variable direction electrodes eachpositioned along a longitudinal axis and having a shape such that eachone diverges from and then approaches its longitudinal axis to intersectit and pass alternately on either side of this axis and plot arepetitive pattern, the divergence shown by a variable directionelectrode with respect to the longitudinal axis having an amplitudevarying as a function of the distance between the variable electrode anda reference position.
 2. An image display screen according to claim 1,wherein the patterns are repeated according to a pitch correspondingsubstantially to the pitch that has to be given to cells parallel to thelongitudinal axes or corresponding to a sub-multiple of this pitch. 3.An image display screen according to claim 1 wherein, between twovariable direction electrodes respectively showing a minimum divergencein amplitude and a maximum divergence in amplitude, the intermediatevariable direction electrodes show values of divergence included betweenthe minimum amplitude and the maximum amplitude and varying in asubstantially gradual manner from one variable direction electrode tothe next.
 4. An image display screen according to claim 1 wherein,between two variable direction electrodes respectively showing a minimumdivergence in amplitude and a maximum divergence in amplitude, theintermediate variable direction electrodes form groups of electrodes,all the electrodes of one and the same group showing values ofdivergence of one and the same amplitude.
 5. An image display screenaccording to claim 4, wherein the amplitudes of the values of divergenceshown by the electrodes vary in a substantially gradual manner from onegroup of electrodes to the next.
 6. An image display screen according toclaim 1, wherein the reference position is a substantially centralposition in a length of a useful zone substantially corresponding to thesurface area occupied by all the electrodes of the array.
 7. An imagedisplay screen according to claim 1, wherein the reference position is aposition located substantially at one end of a length of a useful zonecorresponding substantially to the surface area occupied by all theelectrodes.
 8. An image display screen according to claim 1, wherein thereference position corresponds to a substantially straight electrode. 9.An image display screen according to claim 1, wherein the variabledirection electrodes are separated from the reference position by agroup of straight electrodes.
 10. An image display screen according toclaim 1, wherein the electrodes of the array are positioned on axes thatare all separated from one another by one and the same distance.
 11. Animage display screen according to claim 1, wherein the electrodes of thearray are positioned on axes separated from one another by distanceswhose value is a function of the position of the electrodes with respectto the reference position.
 12. An image display screen according toclaim 4, wherein the electrodes of the array are positioned on axesseparated from one another by distances whose value is a function of thegroup to which the electrode belongs.
 13. An image display screenaccording to claim 1, wherein the image display screen is a plasmapanel.
 14. An image display screen according to the foregoing claim,wherein the variable direction electrodes are borne by a rear plate ofthe plasma panel.
 15. An image display screen according to claim 1,wherein the mask is used to define recesses in a luminophor material.